In the oil and gas exploration and production industry, wells are created by drilling bores from surface to access subsurface hydrocarbon reservoirs. A drill bit is mounted on the end of a string of drill pipe which extends from the surface. The string and bit may be rotated from surface, or the bit may be rotated by a downhole motor. Drilling fluid or “mud” is pumped through the drill string from the surface, to exit the string at the bit. The fluid carries the cuttings produced by the drill bit to surface, through the annulus between the drill string and the bore wall.
The drilled “open” bore is lined with metallic tubing, known as casing or liner, which is secured and sealed in the bore by injecting a cement slurry into the annulus between the liner and the bore wall.
Often, a drilling operation will encounter a “loss zone”, typically a void or an area of porous or fractured strata or a formation in which the in situ pressure regime is lower than in the other exposed zones. When drilling through a loss zone, large volumes of drilling fluid may be lost, at great expense and inconvenience. The loss of drilling fluid may also result in a significant differential fluid pressure between the drill string and the annulus, during drilling and indeed any other downhole operation, which has significant implications for operational safety and operation of conventional downhole tools and devices.
Furthermore, some production zones, such as fractured carbonate reservoirs, act as loss zones. Thus, following completion of a bore, and before oil is produced, much of the drilling fluid lost into the reservoir during drilling must be removed, by “back-producing”, which is both time consuming and expensive.
A further difficulty when a drilled bore crosses a loss zone is that it is difficult to place and successfully cement a conventional bore liner across the zone; the loss zone prevents the cement from being placed across the liner.
As noted above, fractured carbonate reservoirs which are one of the producing formations for oil can act as multiple loss zones. However, to obtain increased production rates, it is desirable that a well accesses a large area of reservoir and thus may intersect many loss zones. Thus, if the first fracture encountered cannot be isolated, by lining and cementing, due to losses, the well cannot be drilled further, and the well can only be produced from this first fracture, limiting production.
A different but related problem is encountered when a drilled bore intersects a relatively high pressure, or “over pressured” zone, which may result in undesirable and possibly uncontrolled flow of fluid into a bore. This flow of fluid into the bore disrupts the normal circulation of drilling fluid, and may have well control implications as the density of the fluid column changes. Furthermore, the reliance on increasing the drilling fluid pressure to retain fluid in the over pressured zone by, for example, using relatively dense drilling fluid, limits the ability to drill the bore beyond the over pressured zone, since fluid losses may occur into other exposed zones which are naturally of a normal or sub-normal pressure regime.
It is among the objectives of embodiments of the present invention to obviate or mitigate these difficulties.